The field of constructing pipelines formed by lengths of pipe in an assembly line comprises a plurality of workstations. In general, the assembly line is arranged on board a laying vessel that launches and lays the pipeline on a bed of a body of water. Each workstation along the assembly line comprises at least one apparatus that is cyclically coupled to the pipeline. Basically, each apparatus is supported by a hoisting device so that apparatus can placed on the pipeline and coupled to the latter, and is decoupled from the pipeline once the apparatus has finished the operations on the pipeline. Since the pipeline is partially immersed in the body of water, the pipeline is inevitably subject to relative movements with respect to the laying vessel. In consequence, the coupling between the work apparatus and the pipeline prevents relative movements between the apparatus and the pipeline and enables the apparatus to perform precision operations on the pipeline. In order to speed up operations along the assembly line, the hoisting device is also connected to the work apparatus when the work apparatus is coupled to the pipeline, so as to be ready to support the apparatus once the work cycle is finished. The hoisting device comprises a flexible elongated member, such as a rope, cable or chain, which is connected to the work apparatus. In the step where the work apparatus is coupled to the pipeline, the flexible elongated member is left slack to allow the apparatus to move together with the pipeline with respect to the laying vessel without stressing the hoisting device.
In the specific field of pipeline laying, the pipeline is progressively released as the pipeline is assembled. The advancement of the pipeline is controlled by braking devices that progressively release the pipeline according to the work cycles of the assembly line as disclosed in PCT Patent Application No. WO 2012/140631. Basically, the braking devices grip the pipeline with tracks or clamps, and alternate steps of locking the pipeline with steps of controlled advancement or release of the pipeline. The natural descent or advancement of the pipeline with respect to the laying vessel is caused by the slope and/or forward movement of the laying vessel. In exceptional situations, especially when the weather and sea conditions are particularly critical, friction could be lost between the braking devices and the pipeline, with consequent loss of control over the pipeline. An event of this type is very rare because the safety systems on board the laying vessel are redundant, as is the sizing of the braking devices. Nevertheless, the loss of the pipeline would be a particularly destructive event if the work apparatuses are coupled to the pipeline because the work apparatuses could drag the respective hoisting devices with them, which are in turn constrained to a load-bearing structure of the assembly line.
Therefore, the potential consequences of losing the pipeline would not just possibly produce disastrous effects on the material structures on board the laying vessel, but, due to the above-described domino effect, could place the safety of the crew at potential risk.